Following the excessive consumption of the mineral fuels such as the petroleum and natural gas, the prices of fuels are continuously increased, and the influences towards the environment caused by the excessive consumption of the mineral fuels are more and more significantly at the same time. To meet the economy and environmental protection requirements, the alternative energy sources are sought globally, and employing the renewable energy (such as wind energy, solar energy and so on) to generate the electric power is the most important development direction. Although being developed for many years, there are still drawbacks currently in employing solar cell to generate the electric power such as excessive costs and lower efficiency. FIG. 1 shows a schematic circuit diagram of a conventional non-isolated inverter circuit. In FIG. 1, the inverter circuit includes a regulator connected to an input DC power source having a positive and a negative terminals and providing a DC voltage Vin, a capacitor Cp having a first terminal coupled to the negative terminal of the DC power source and a second terminal coupled to the ground, a first to a fourth switches, S1-S4, employed to form a single-phase full-bridge inverter, a first inductor having a first terminal coupled to a first middle point being connected to the first switch S1 and the second switch S2, and a second terminal electrically connected to an AC power source being connected to a second middle point of switches S3 and S4, and the voltage and the current of the AC power source are VG and IG respectively. FIG. 1 is also a circuit diagram of a solar cell power system. For example, an output voltage of a solar cell board, Vin, is outputted to an input terminal of the regulator, the output of the regulator reaches a stable DC voltage such as 400V after voltage boosting performed by the regulator, the stable DC voltage is converted to an AC voltage, VG, by an inverter, and VG is outputted to the power network. The regulator in FIG. 1 is well-known to the public, there are various methods to realize the regulator, and they are omitted here. The inverter circuit in FIG. 1 employs the non-isolated type configuration, so a transformer is saved, and the first to the fourth switches, S1-S4, are all power switches and could be either MOSFET, or insulated-gate bipolar transistor (IGBT) etc. If IGBTs are employed, the turn-on losses of which are quite large since the turn-on voltage drop is pretty high, usually exceeding 2V, that will cause the lower efficiency of the inverter employing the IGBT, and the efficiency is lower than 97% according to the existing technologies. If four MOSFETs are employed instead of four IGBTs, the turn-on losses of MOSFETs are decreased, but the loss of the body diode included in the MOSFET at the stage of freewheeling current is increased. S3 is constantly turned on and S1 is used as a PWM switch to regulate the output voltage when the AC output voltage VG is in the positive-half cycle and is positive. The current flows through S1-L1-L3 when S1 is turned on and the current flows through the body diode of S2-L1-S3 when S1 is turned off, results in quite large loss due to the poor characteristic of the body diode of S2, and is disadvantageous to the increase of the efficiency. When the AC output voltage VG is in the negative-half cycle and is negative, S4 is constantly turned on and there is also a stage of freewheeling current of the body diode of S1 while S2 is used as a PWM switch, and also results in a great deal of loss.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived an inverter circuit having a relatively higher efficiency.